Accept svg file content sent by postMessage
Merge pull request #104 from byte-foundry/svg_message

This is really cool - thanks!  I know SVG isn't the best font format, but it does fit into other workflows very very well, like creating outlines in Illustrator or Inkscape and re-using them in Glyphr Studio.

I'm putting the finishing touches on Beta 5, it should be released very soon!